Plants have always had a major role in medicine and public health, and the pharmacological potential of plants has been one of the major reasons for preserving biodiversity. While their value has been appreciated in a qualitative sense, national policy-making, which relies on benefit-cost analyses, requires a quantitative valuation. In addition to its utility in providing a basis for estimating the economic consequences of the loss of biodiversity, the pharmacological potential of plants may also be one of the most easily estimated indicators of ecosystem value. The market value of plant-based prescription drugs in the United States in 1990 is estimated to be $15.5 billion (retail). Given current extinction rates and the estimated likelihood of developing new, marketable pharmaceutical products, the annual market benefit in the United States expected to be foregone as a result of plant extinction is about $150 million. The present value in 1991 of the foregone benefit in 2050 is about $3 billion--which includes only market value. Methodological issues related to the estimation of the economic value of plant-based pharmaceuticals are discussed, and the economic value of plant-based anti-cancer drugs is estimated.